1. Field of the Invention
The present invention pertains to the recovery of ethylene from light gases at low temperature, and more particularly to an improved mixed refrigeration system comprising (1) methane (2) ethylene and/or ethane, and (3) propylene and/or propane to provide more efficient refrigeration for such recovery.
2. Description of the Prior Art
Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple refrigerants are utilized in a single refrigeration system to provide refrigeration covering a wider range of temperatures, enabling one mixed refrigeration system to replace multiple pure component cascade refrigeration systems. These mixed refrigeration systems have found widespread use in base load liquid natural gas plants.
Gaumer et al., U.S. Pat. No. 3,593,535 (Jul. 20, 1971) and U.S. Pat. No. 3,763,658 (Oct. 9, 1973); Stockmann et al., U.S. Pat. No. 6,253,574 (Jul. 3, 2001); and Roberts et al., U.S. Pat. No. 6,347,531 (Feb. 19, 2002); and Kinard et al., “Mixed Refrigerant Cascade Cycles for LNG,” Chemical Engineering Progress; Vol. 69, No. 1, pages 56–61 (January 1973), disclose methods for liquefying gas, especially natural gas, that employ multicomponent refrigeration systems. None of these methods are employed for the recovery of ethylene and separation therefrom of methane. Bauer, U.S. Pat. No. 5,430,223 (Jul. 4, 1995) discloses a refrigeration system for use in the separation of higher hydrocarbons from their gas mixture with lower boiling components, but not the separation of methane from ethylene.
Ethylene plants require refrigeration to separate out desired products from the cracking heater effluent. Typically, a C3 refrigerant, usually propylene, and a C2 refrigerant, typically ethylene, are used. Often, particularly in systems using low pressure demethanizers where lower temperatures are required, a separate methane refrigeration system is also employed. Thus, three separate refrigeration systems are required, cascading from the lowest temperature to the highest. Three compressor and driver systems complete with suction drums, separate exchangers, piping, etc., are required. Also, a methane refrigeration cycle often requires reciprocating compressors which can partially offset any capital cost savings resulting from the use of low pressure demethanizers. Hence, the use of a mixed refrigerant system is highly desirable.
Howard et al., U.S. Pat. No. 5,379,597 (Jan. 10, 1995) discloses a method for recovering ethylene from a feed gas containing ethylene, hydrogen and C1 to C3 hydrocarbons, which includes the steps of compressing and cooling the feed gas to condense a portion thereof, fractionating the condensed feed gas liquids in one or more demethanizer columns to recover a light overhead product comprising chiefly hydrogen and methane, and fractionating the one or more demethanizer column bottoms streams to recover an ethylene product and streams containing C2 and heavier hydrocarbons. The refrigeration cycle employed for this recovery involves condensing and subcooling a mixed refrigerant vapor. The resulting subcooled liquid is split in two portions, each of which is subsequently flashed. One such portion is at least partially vaporized in the demethanizer column overhead condenser to provide reflux to that column. The other such portion is at least partially vaporized in cooling the feed gas. The resulting vapor refrigerant portions are recombined.
Wei, published U.S. patent application No. U.S. 2002/0174679 A1, published on Nov. 28, 2002 discloses a refrigeration system for an ethylene plant that comprises a tertiary refrigerant containing methane, ethylene and propylene. In the closed loop system, a portion of the constant composition refrigerant from the compressor is separated into a methane-rich vapor portion and a propylene-rich liquid portion. The various refrigerant streams are then used to cool the charge gas to separate the C2 and heavier hydrocarbons from the hydrogen and methane. The separated refrigerant streams are then recombined to form the constant composition before recycle to the compressor.
It is highly desirable to provide refrigeration using a mixed refrigeration system that provides refrigeration at a lower temperature and a greater degree of control over the relative cooling duties of the heat exchangers.